In a typical engine, 35-40% of the fuel energy is released in the form of exhaust gas heat, which can be as high as 500-600° C. for high speed engines. Exhaust driven sorption heat pumps, like absorption heat pumps, metal hydride heat pumps, and adsorption heat pumps, are being in use to recover heat from exhaust gases and convert the same to provide cooling and/or heating inside the vehicle. The outlet temperature of the exhaust gas after heat recovery in the heat pump can be up to 200° C.
A major issue in relation to typical metal hydride heat pumps is that ideally, a sorption bed temperature of such a pump is in the range of about 80° C. to 200° C., while an exhaust temperature requirement is of about 100° C. to 250° C. depending on heat rejection temperature and cooling temperature requirements. Further, exhaust gases have dust and soot, which settle on a heat exchanging surface of a heat exchanger of a metal hydride heat pump. Also, when exhaust gases are cooled to temperatures below 250° C., condensation of acid exhaust gas constituents as well as deposits of exhaust gas constituents occur, which leads to the clogging of the heat exchanger, thereby hampering its efficiency. The exhaust gas condensate is corrosive and is produced when the temperature drops below the dew point. This corrosive exhaust gas condensate will eventually produce corrosive effects in components of the metal hydride heat pump such as fins and tube surfaces.
FIG. 1 illustrates a conventional sorption heat pump 100 for exhaust gas heat recovery. A desorber 3 of the sorption heat pump 100 receives external heat input for desorption of a refrigerant material from a sorbent media. In a typical metal hydride heat pump, hydrogen is used as the refrigerant material while a metal hydride alloy acts as the sorbent media. The desorber 3 receives external heat input in the form of a high temperature exhaust gas. An exhaust gas inlet 1 to the desorber 3 provides exhaust gases from an engine of temperatures up to 500-600° C. This exhaust gas is cooled by the sorbent media, i.e., the metal hydride alloy. A desorber outlet stream 4 of the exhaust gas that is released to the atmosphere via for example a chimney is typically at 100-200° C. The exhaust gas stream passes through a plurality of passes 5 and a number of flow reversals 6 to achieve a better heat transfer rate. In the method as described above, exhaust gases at a high temperature are used directly as input. The arrangement has certain limitations which are as follows.
The direct use of exhaust gases having temperature as high as 500-600° C. can lead to overheating of heat transfer surfaces like tube and fins of the heat exchanger, whereas typical required temperatures for the operation of the heat pump is only 100-250° C. Further, as the metal hydride heat pump operation is cyclic in nature and requires alternate heating and cooling of the heat exchanger and sorption materials in each cycle, overheating of the heat exchanging surfaces results in a higher thermal inertia of metal hydride and the heat pump. Higher thermal inertia is highly undesirable for the performance of the metal hydride heat pump and the adsorption heat pump. The increased thermal inertia reduces cooling capacity and COP (co-efficient of performance) of the whole of the system.
Further, direct use of the exhaust gases results in higher temperature difference of up to 400° C. across the heat exchanger. This results in higher variation in the volumetric flow, hence large variation of velocity of the exhaust gas in the heat exchanger. This reduces heat transfer rate, thus resulting in higher size of the heat exchanger.
Furthermore, direct use of the exhaust gases requires several passes to be provided to maintain velocity in the heat exchanger. The exhaust flow quantity is small compared to heat content and a size of the heat exchanger. More the number of passes and flow reversals, higher the pressure drop through the heat exchanger. Further, small quantity of exhaust gases in the heat exchanger having a relatively large size leads to a non-uniform distribution of the exhaust gases over the heat exchanger, which results in reduced performance of the heat exchanger and the whole of the system.
In addition, there is a higher differential expansion in the heat exchanger due to the use of high temperature direct exhaust gases. This may sometimes result in failure of tubes, reducing the reliability of the heat exchanger and the system itself. Also, there is possibility of thermal creep failure as the whole operation is cyclic and alternates between ambient to exhaust temperature. In conventional systems, a cyclic temperature difference will be typically up to 500° C. and due to a reduced cyclic temperature difference, thermal creep will be higher. This will result in reduced life and lower number of cycles of operation of the system.
Moreover, to construct a heat exchanger that uses high temperature exhaust gases, the material needs to be suitably chosen. This may make the heat exchanger expensive.
Published US patent document US20140047853 discloses a motor vehicle climate control system that implements two heat transfer fluid (HTF) circuits—a cold HTF and a hot HTF circuit. Exhaust gas heat recovery is used in the hot HTF circuit for recovering heat and transferring it to an adsorption driven heat pump system. It is to be noted here that implementation of a circuit like this requires additional heat exchangers for a) transferring exhaust gas heat to a heat transfer fluid; and b) leading away heat of adsorption via the heat transfer fluid to the atmosphere. Use of such circuit for heat transfer also requires additional pumps for circulation of fluid and multiple valves for a changeover of the cycle. This makes the whole system more complex, consuming more auxiliary power and also less reliable due to a number of moving parts. The system will also be bulky and expensive.
Another US published patent document US 20050274493 A1 discloses a metal hydride-based vehicular exhaust cooler that works at about 1000° F. (appx. 600° C.) exhaust heat temperature. This system uses eight valves and a prolonged operation at a high temperature, which leads to thermal inertia and reduced performance. It does not address issues relating to overheating of heat transfer surfaces, higher thermal inertia, and issues related to performance and efficiency.
Thus, there is a need to minimize the dust and soot and increase the overall efficiency as well as better heat transfer rate of heat regeneration module.